Rationalism and empiricism were two philosophical schools of the 17th and 18th centuries, which expressed opposing opinions on some topics, including knowledge. Although the debate between the rationalist and empiricist schools had no bearing on the study of psychology at the time, it contributed greatly to facilitating the possibility of establishing the discipline of psychology. This essay will describe the empiricist and rationalist debate and relate this debate to the history of psychology. The debate between rationalist and empiricist philosophers examines the nature of knowledge and, in particular, how we obtain this knowledge. Rationalists and empiricists have opposing, and sometimes mutually exclusive, views of how knowledge is obtained. Rationalism is based on the assumption that all human beings are innately rational. French and German rationalist philosophers, such as Decartes, Spinoza, Leibniz, and Kant, believed that fundamental metaphysical questions could only be answered by reason. In his work Discourse on Method, Decartes attempted to arrive at a set of fundamental principles, and thus to arrive at true knowledge; to do this he has methodologically rejected everything he can doubt. Decartes summarized his conclusion by saying “I think therefore I am” (Decartes, 1637); he concluded that only thought exists, and since thought cannot be separated from him, he also concluded that it exists. This conclusion that only the existence of thought cannot be doubted has led to the belief that reason and thought are the nature of the soul and that human beings are fundamentally rational, is the foundation of rationalist thought. According to rationalist philosophers, reason is what separates man... the medium of paper... a Body. (G. MacDonald Ross, trans.). Retrieved December 1, 2011: http://www.philosophy.leeds.ac.uk/GMR/hmp/texts/modern/descartes /body/body.htmlGeorge Boeree, C. (2000). Modern philosophy: the Enlightenment. Retrieved December 1, 2011: http://webspace.ship.edu/cgboer/empvsrat.htmlKumar Singh, A. (1991). The complete history of psychology. Delhi, India: Motilal Banarsidass Publishers PVT. Retrieved December 1, 2011: http://books.google.ieLeibniz, G.W. (1714). Monadology. (R. Latta, trans.). Retrieved December 1, 2011: http://www.rbjones.com/rbjpub/philos/classics/leibniz/monad.htmLocke, J. (1690). An essay on human understanding. Retrieved December 1, 2011: http://www.gutenberg.org/catalog/world/readfile?fk_files=1477354Richards, G. (2010). Putting psychology in its place: Critical historical perspectives (3rd ed.). London: Routledge.